Process of containing and fixing fission products



Sept. 19, 1961 E. J. EVANS 3,000,072

PROCESS oF CONTAINING AND FIXING FIssIoN PRODUCTS Filed Aug. 2o, 1959 P/u/fger Nv 'TOR United States Patent 3,000,072 PROCESS F CONTAYLNVING AND FIXING FISSION PRODUCTS Edward J. Evans, Qttawa, Ontario', Canada, assignor to Atomic Energy of Canada Limited, Ottawa, Ontario, Canada, a company Filed Aug. 2.0, 1959, Ser. No. 835,127 Claims. (Cl. 25--156) This invention relates to a process of incorporating fission products into synthetic mica for the purpose of safe disposal of the -iission products in a solid having longterm weathering properties and to provide a product in form as a radiation source.

More particularly the object of the invention is to in corporate the fission products by solid state reaction in a synthetic mica to form a long-term weatherproof solid.

The process of the invention will be described with reference to lthe accompanying ydrawing in which,

FIGURE l is an illustrative flow sheet.

FIGURE 2 is a sectional View of a crucible for forming the synthetic mica.

FIGURE 3 illustrates a pellet forming device, and

FIGURE 4 is a diagrammatic view, partly in section, indicating the manner of coating.

In the ow sheet of FIGURE 1 the term waste means the product formed upon dissolving a sheathed fuel rod, used in a nuclear reactor, in nitric acid. It may be one or more separated radioactive isotopes. The term mica bate means the compounds which combine to form a uor-phlogopite mica. Potassium uor-phlogopite is the preferred mica. KMggAlSiaOmFz represents the mole ratio of the elements used in the batch. It is known that the layers or plates of such mica are separated and held together by the potassium. In the operation of the present process, as the batch material is heated with the fission product material at a temperature not less than 900 C. and less than the melting point of the batch material the fission products are incorporated in the mica. The melting point of the batch material may be as high as l450 C. In the structure of the synthetic mica formed it appears that iission products cesium and strontium take the place of the potassium and/or magnesium as it appears in natural iluor-phlogopite. The resulting mass containing the fission products is readily pulverized.

As shown in the dow sheet the mixture of mica batch and iission products material is dried and heated to form the synthetic mica. A ,temperature of l000 C. has proven satisfactory. As stated the temperature is less than the melting point of the batch material and it should not be less than about 900 C. The incorporation of fthe fission products in the synthetic mica thus takes place in the solids state. FIGURE 2 illustrates a crucble in which the synthetic mica is formed.

The mass is then pulverized and formed into pellets, as illustrated in FIGURE 3. When the rnass was heated at a temperature of about 1275 C. a pressure of about 1000 p.s.i. was effective. The pelletizing apparatus is preferably contaned in a dry box in which an inert atmosphere is maintained to avoid deterioration of the mould.

In order to insure against release of iission products from the pellets a coating of inactive material is hot pressed about the pellets as illustrated in FIGURE 4. As inactive material for the coating the following may be used: synthetic mica, natural potassium feldspar, natural feldspar treated with hydrochloric acid (a hydrogen substituted feldspar) montmorillonite, vermiculite, kaolinite,

The operation of the process as used ismore particullarly described as follows.

The mica batch was formed by compounds to provide the correct molar proportions for formation of the desired synthetic mica, as illustrated by the formula In practice an extra one-half mole of uon'de is used to compensate for losses by volatiliza-tion. The iission product was mixed with a measured quantity oi the mica batch and the mixture dried under Ian infra-red lamp. The dried mixture was heated in a Crucible, as illustrated in FIGURE 2, at a temperature of l000 C. to cause the solid state reaction incorporating the -ssion product in the synthetic mica formed.

'1`he product was crushed and formed into pellets, as illustrated in FIGURE 3, by heating the pulverized material to a temperature of about 1275 C. under a pres# sure of about 1000 p.s.i. l

In the preferred procedure illustrated i-n FIGURE 4 the pulverized mass was formed into pellets and coated with the non-radioactive material. As shown the mould contains -a liner which prevents the pellet from adhering to the mould and plunger. A molybdenum liner is eiective. Inactive material is placed in the bottom of the mould, an open bottom wax cylinder is centrally located, lled with the radioactive synthetic mica powder, and completely surrounded with inactive material to provide a coating of the desired thickness on the pellet. The mould is heated at a temperature of not substantially less than 900 C. and less than the melting point of the syn# thetic mica. The melting point of uor-phlogopite is 'about 1350 C. The mica and coating material is then subjected to a pressure of not substantially less than 1000 p.s.i. to produce a coated pelletized synthetic mica. The coating prevents release of iission products `from the pellets. Y

Pellets, containing radioactive cesium coated with inlactirve acideleached feldspar prepared by hot pressing at 1175 C. at 1000 p.s.i. permitted cesium to leach at a rate of about 1x10-7% per `day from distilled water after the rst .ten days of leaching. Identical pellets without the coating leached at the rate of 5x10-5% per day following an initial twenty day leaching period. Thus the coating gave additional protection of more than one hundredfold.

What is claimed is:

l. A method of containing and fixing iission products which comprises mixing fission product material with the constituents substantially in molar proportions to form synthetic mica, heating the mixture at a temperature below its melting point and not substantially less than 900 C., pulverizng the product and compressing the pulverized product into pellets at a temperature below the melting point of the mica and a pressure of not substantially less than 1000 p.s.i.

2. The process defined in claim l wherein the pellets are coated under pressure with a non-radioactive siliceous material.

3. The process defined in claim l wherein the synthetic mica is uor-phlogopite.

4. A process of containing and fixing iission products which comprises mixing fission product material with constituents to form uor-phlogopite, heating the mix- 7o ture at a temperature below its melting point and not sub- Y Patented Sept. 19,1961A I 4 stantally less than 900 C. to form synthetic mica, pul- References Cited in the le of this patent verizing the product, surrounding portions of the pulverized mica with non-radioactive siliceous material and sub- UNITED STATES PATENTS jecting the coated portions to a pressure not substantially 2,918,700 Hatch DeC- 29, 1959 less than 1000 p.s.i. at a temperature below the lnelgiugV 5 2,918,717 SU'uXIleSS et a1 D56 29, 1959 point of the mlca to form pellets. j OTHER'REFERENCES 5. The process defined in claim 4 wherein an excess of. uoride is incorporated in the constituents forming lnhe synthetic mica. vol. 18, September 1958, pp. 154-155, 441.

U.N.l `Publication, Peaceful Uses of Atomic Energy, 

1. A METHOD OF CONTAINING AND FIXING FISSION PRODUCTS WHICH COMPRISES MIXING FISSION PRODUCT MATERIAL WITH THE CONSTITUENTS SUBSTANTIALLY IN MOLAR PROPORTIONS TO FORM SYNTHETIC MICA, HEATING THE MIXTURE AT A TEMPERATURE BELOW ITS MELTING POINT AND NOT SUBSTANTIALLY LESS THAN 900*C., PULVERIZING THE PRODUCT AND COMPRESSING THE PULVERIZED PRODUCT INTO PELLETS AT A TEMPERATURE BELOW 